Modern societies are critically dependent on energy to maintain their standards of living and economic viabilities. All aspects of modern life, ranging from the generation of electricity to the powering of automobiles, require the consumption of energy. Conventional fossil fuels are primarily used to meet the energy needs of today's societies. As more societies modernize and existing modern societies expand, the consumption of energy continues to increase at ever growing rates. The increased worldwide use of fossil fuels is creating a number of problems. First, fossil fuels are a finite resource and concern is growing that fossil fuels will become fully depleted in the foreseeable future. Scarcity raises the possibility that escalating costs could destabilize economies as well as the likelihood that nations will go to war over the remaining reserves. Second, fossil fuels are highly polluting. The greater combustion of fossil fuels has prompted recognition of global warming and the dangers it poses to the stability of the earth's ecosystem. In addition to greenhouse gases, the combustion of fossil fuels produces soot and other pollutants that are injurious to humans and animals. In order to prevent the increasingly deleterious effects of fossil fuels, new energy sources are needed.
The desired attributes of a new fuel or energy source include low cost, plentiful supply, renewability, safety, and environmental compatibility. Hydrogen is currently the best prospect for these desired attributes and offers the potential to greatly reduce our dependence on conventional fossil fuels. Hydrogen is the most ubiquitous element in the universe and, if realized, offers an inexhaustible fuel source to meet the increasing energy demands of the world. Hydrogen is available from a variety of sources including coal, natural gas, hydrocarbons in general, organic materials, inorganic hydrides and water. These sources are geographically well distributed around the world and accessible to most of the world's population without the need to import. In addition to being plentiful and widely available, hydrogen is also a clean fuel source. Combustion of hydrogen produces water as a by-product. Utilization of hydrogen as a fuel source thus avoids the unwanted generation of the carbon and nitrogen based greenhouse gases that are responsible for global warming as well as the unwanted production of soot and other carbon based pollutants in industrial manufacturing. Hydrogen truly is a green energy source.
The realization of hydrogen as a ubiquitous source of energy ultimately depends on its economic feasibility. Economically viable methods for producing hydrogen as well as efficient means for storing, transferring, and consuming hydrogen, are needed. Chemical and electrochemical methods have been proposed for the production of hydrogen. The most readily available chemical feedstocks for hydrogen are organic compounds, primarily hydrocarbons and oxygenated hydrocarbons. Common methods for obtaining hydrogen from hydrocarbons and oxygenated hydrocarbons are dehydrogenation reactions and oxidation reactions.
Steam reformation and the electrochemical generation of hydrogen from water through electrolysis are two common strategies currently used for producing hydrogen. Both strategies, however, suffer from drawbacks that limit their practical application and/or cost effectiveness. Steam reformation reactions are endothermic at room temperature and therefore require heating. Temperatures of a few to several hundred degrees are needed to realize acceptable reaction rates. These temperatures are costly to provide, impose special requirements on the materials used to construct the reactors, and limit the range of applications. Steam reformation reactions also occur in the gas phase, which means that hydrogen must be recovered from a mixture of gases through a separation process that adds cost and complexity to the reformation process. Steam reformation also leads to the production of the undesirable greenhouse gases CO2 and/or CO as by-products. Water electrolysis has not been widely used in practice because high expenditures of electrical energy are required to effect water electrolysis. The water electrolysis reaction requires a high minimum voltage to initiate and an even higher voltage to achieve practical rates of hydrogen production. The high voltage leads to high electrical energy costs for the water electrolysis reaction and has inhibited its widespread use.
In the co-pending parent applications (U.S. patent application Ser. Nos. 09/929,940 U.S. Pat. No. 6,607,707, and Ser. No. 10/321,935, U.S. Pat. No. 6,890,419, the disclosures of which are incorporated by reference herein), the instant inventors considered the production of hydrogen from hydrocarbons and oxygenated hydrocarbons. In U.S. patent application Ser. No. 09/929,940, the instant inventors considered the production of hydrogen through reactions of hydrocarbons and oxygenated hydrocarbons with a base. Using a thermodynamic analysis, the instant inventors determined that reactions of many hydrocarbons and oxygenated hydrocarbons react spontaneously with a base or basic aqueous solution to form hydrogen gas at particular reaction conditions, while the same hydrocarbons and oxygenated hydrocarbons react non-spontaneously in conventional steam reformation processes at the same reaction conditions. Inclusion of a base was thus shown to facilitate the formation of hydrogen from many hydrocarbons and oxygenated hydrocarbons and enabled the production of hydrogen at less extreme conditions than those normally encountered in steam reformation reactions.
Representative hydrogen producing reactions disclosed in co-pending U.S. patent application Ser. No. 09/929,940 include the reactions of methanol in the presence of a base shown below:CH3OH+OH−+H2O3H2+HCO3−CH3OH+2OH−3H2+CO32−As discussed in the co-pending U.S. application Ser. No. 09/929,940, U.S. Pat. No. 6,607,707, both reactions may occur separately or simultaneously depending on the reaction conditions. The inventors showed that hydrogen was produced from a liquid phase mixture of methanol and a base and that hydrogen was the only gaseous product formed, thereby obviating the need for the gas phase separation required for conventional steam reformation processes. The required reaction temperature was less than the boiling point of the mixture and required only a modest input of energy to effect. Analogous reactions with other hydrocarbons and oxygenated hydrocarbons were also disclosed.
In co-pending U.S. patent application Ser. No. 10/321,935, U.S. Pat. No. 6,890,419, the instant inventors considered electrochemical methods to promote the production of hydrogen from organic substances in the presence of water (e.g. acidic solution) and/or a base. They showed that electrochemical reactions of organic substances with water to produce hydrogen require lower electrochemical cell voltages than water electrolysis. They also showed that electrochemical reactions of organic substances in the presence of an acid or base require low electrochemical cell voltages at room temperature. In some embodiments, hydrogen production reactions of organic substances were shown to occur spontaneously at room temperature in an electrochemical reaction and were accelerated by heating. In other embodiments, hydrogen production reactions of organic substances were shown to occur spontaneously at room temperature without applying a voltage and were accelerated by providing a voltage.
A representative example of a hydrogen producing electrochemical reaction disclosed in co-pending U.S. application Ser. No. 10/321,935, U.S. Pat. No. 6,890,419 is a reaction of methanol with a base in the presence of an electrochemical potential. The corresponding electrochemical reactions are shown below: Methanol may also react with two equivalents of hydroxide ion to produce hydrogen gas according to the following electrochemical reactions: The amount of base present in the reaction mixture influences whether methanol reacts primarily with one or two equivalents of hydroxide ion in the overall reaction. In principle, both reactions can occur simultaneously and in practice, the specific reaction conditions determine whether one overall reaction is more important than the other overall reaction. The instant inventors also showed that the electrochemical reactions of methanol with one or two equivalents of hydroxide ion occur spontaneously at room temperature and that application of an electrochemical potential increased the rate of hydrogen production from each reaction. Similarly beneficial effects were disclosed for electrochemical reactions of other organic substances.
Bases suitable for the reactions disclosed in the co-pending parent applications (U.S. patent application Ser. Nos. 09/929,940, Pat. 6,607,707, and Ser, No. 10/321,935, Pat. 6,890,419) are compounds that provide hydroxide ions. Metal hydroxides are the preferred bases. Representative metal hydroxides include alkali metal hydroxides (e.g. NaOH, KOH etc.) alkaline earth metal hydroxides (e.g. Ca(OH)2, Mg(OH)2, etc.), transition metal hydroxides, post-transition metal hydroxides and rare earth hydroxides. Non-metal hydroxides such as ammonium hydroxide may also be used.
Realization of the beneficial properties of reactions that produce hydrogen from organic substances and bases requires a system level consideration of the costs and overall efficiency of the reactions. In addition to energy inputs and raw materials, consideration of the disposal or utilization of by-products must be made. Many of the reactions of a base with hydrocarbons, oxygenated hydrocarbons and other organic substances discussed in the co-pending parent applications involve the formation of the carbonate ion (CO32−) or bicarbonate ion (HCO3−) as a by-product. In order to enhance the efficiency and economic viability of these reactions, it is necessary to devise ways to effectively dispense with the carbonate ion and/or bicarbonate ion by-products. It is particularly desirable to dispense with by-products in such a way as to avoid the production of environmentally harmful gases and/or in such a way as to regenerate the base reactant.